The safest suit for maximum fire protection is a loose fitting one.
A suit's fire protection capability is directly related to the fit of the garment. A properly fitted racing suit will have small air gaps in between the body and the garment. A suit worn too tightly will compress these air gaps- allowing heat to reach the skin faster.
An additional consideration should be the position the garment will be worn in. Specifically, the clamshell orientation of the driving position requires additional material in order to prevent tugging the wearer. This is especially important when wearing a one piece suit. A racing suit that fits well while standing may not fit comfortably when seated and harnessed into a racing seat. It is important to account for this when selecting a suit: ensure you are the lower end of the range for a particular size to ensure that there will be enough material to prevent the suit from tugging when seated.